Quadcopters can continuously observe ocean surface with high spatial resolution from relatively low altitude, albeit with certain limitations of their usage. Remote sensing from quadcopters provides unprecedented ability to study small river plumes formed in the coastal sea. The main goal of the current work is to describe structure and temporal variability of small river plumes on small spatial and temporal scales, which are limitedly covered by previous studies. We analyze optical imagery and video records acquired by quadcopters and accompanied by synchronous in situ measurements and satellite observations within the Kodor and Bzyp plumes, which are located in the northeastern part of the Black Sea. We describe extremely rapid response of these river plume to energetic rotating coastal eddies. We reveal several types of internal waves within these river plumes, measure their spatial and dynamical characteristics, and identify mechanisms of their generation. We suggest a new mechanism of formation of undulate fronts between small river plumes and ambient sea, which induces energetic lateral mixing across these fronts. The results reported in this study are addressed for the first time as previous related works were mainly limited by low spatial and/or temporal resolution of in situ measurements and satellite imagery.

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Wind forcing is the main driver of river plume dynamics. Direction and magnitude of wind determine position, shape, and size of a river plume. The response of river plumes on wind forcing was simulated in many numerical modeling studies; however, in situ measurements of this process are still very scarce. In this study, we report the first direct measurements of frontal movement of a small river plume under variable wind forcing conditions. Using quadcopters, we performed nearly continuous daytime aerial observations of the Bzyb river plume located in the non-tidal Black Sea. The aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing. We assessed spreading patterns of the plume and evaluated movement velocity of its outer border with unprecedentedly high spatial (∼10 m) and temporal (∼1 min) resolution, which was not available in previous studies based on in situ measurements and satellite observations. Based on the collected data, we evaluated the time of response of plume spreading dynamics on changes in wind forcing conditions. The advection velocity of the outer plume border shows linear relation to wind speed with very small response time (10–20 min). The reversal between upstream/downstream plume spreading occurs during several hours under moderate wind forcing conditions. These reversals involve only near-field part of the plume, which cause detachment of the far-field part of the plume. The obtained results are crucial for understanding and simulating spreading dynamics of small river plumes worldwide.

This study is focused on concentric rings, which are regularly observed by remote sensing of small river plumes located in different regions worldwide. We report new aerial observations of these features obtained by quadcopters and supported by synchronous in situ measurements, which were collected during the recent field survey at the Bzyb river plume in the eastern part of the Black Sea. Joint analysis of remote sensing imagery and in situ data suggest that the observed concentric rings are surface manifestations of high-frequency internal waves generated in the vicinity of the river mouth. The obtained results demonstrate that the propagation of these waves does not induce offshore material transport within the plume induced by shear instability, which was hypothesized in a recent numerical modeling study of this process. We provide an explanation for the appearance of misleading material features in the numerical simulations discussed above. Finally, we discuss directions for future research of high-frequency internal waves generated in small river plumes.

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The interfaces between small river plumes and ambient seawater have extremely sharp horizontal and vertical salinity gradients, often accompanied by velocity shear. It results in formation of instabilities at the lateral borders of small plumes. In this study, we use high-resolution aerial remote sensing supported by in situ measurements to study these instabilities. We describe their spatial and temporal characteristics and then reconstruct their relation to density gradient and velocity shear. We report that Rayleigh–Taylor instabilities, with spatial scales ~5–50 m, are common features of the sharp plume-sea interfaces and their sizes are proportional to the Atwood number determined by the cross-shore density gradient. Kelvin–Helmholtz instabilities have a smaller size (~3–7 m) and are formed at the plume border in case of velocity shear >20–30 cm/s. Both instabilities induce mass transport across the plume-sea interfaces, which modifies salinity structure of the plume borders and induces lateral mixing of small river plumes. In addition, aerial observations revealed wind-driven Stokes transport across the sharp plume-sea interface, which occurs in the shallow (~2–3 cm) surface layer. This process limitedly affects salinity structure and mixing at the plume border, however, it could be an important issue for the spread of river-borne floating particles in the ocean.

В статье приведены результаты диатомового анализа голоценовых отложений, полученных из кутовой части Ругозерской губы Кандалакшского залива Белого моря. В исследованных осадках выделены 5 диатомовых зон, отражающих основные этапы эволюции природной среды, начиная с середины атлантического периода. In this paper we present the results of diatom study of Holocene sediments from the inner part of the Rugozerskaya Bay (Kandalaksha Bay of the White Sea). Five diatom ecological zones were established, that clarify the main stages of the natural environment evolution, since the second part of the Atlantic.

<p>Wind forcing is the main driver of river plume dynamics. Direction and magnitude of wind determine position, shape, and size of a river plume. The response of river plumes on wind forcing was simulated in many numerical modeling studies; however, in situ measurements of this process are still very scarce. In this study, we report the first direct measurements of frontal movement of a small river plume under variable wind forcing conditions. Using quadcopters, we performed nearly continuous daytime aerial observations of the Bzyb river plume located in the non-tidal Black Sea. The aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing. We assessed spreading patterns of the plume and evaluated movement velocity of its outer border with unprecedentedly high spatial (~ 10 m) and temporal (~ 1 minute) resolution, which was not available in previous studies based on in situ measurements and satellite observations. Based on the collected data, we evaluated the time of response of plume spreading dynamics on changes in wind forcing conditions. The advection velocity of the outer plume border shows linear relation to wind speed with very small response time (10-20 minutes). The reversal between upstream/downstream plume spreading occurs during several hours under moderate wind forcing conditions. These reversals involve only near-field part of the plume, which cause detachment of the far-field part of the plume. The obtained results are crucial for understanding and simulating spreading dynamics of small river plumes worldwide.</p>